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TIMECOUNTERS(4)		 BSD Kernel Interfaces Manual	       TIMECOUNTERS(4)

NAME
     timecounters -- kernel time counters subsystem

SYNOPSIS
     The kernel	uses several types of time-related devices, such as: real time
     clocks, time counters and event timers.  Real time	clocks are responsible
     for tracking real world time, mostly when the system is down.  Time coun-
     ters are responsible for tracking purposes, when the system is running.
     Event timers are responsible for generating interrupts at a specified
     time or periodically, to run different time-based events.	This page is
     about the second.

DESCRIPTION
     Time counters are the lowest level	of time	tracking in the	kernel.	 They
     provide monotonically increasing timestamps with known width and update
     frequency.	 They can overflow, drift, etc and so in raw form can be used
     only in very limited performance-critical places like the process sched-
     uler.

     More usable time is created by scaling the	values read from the selected
     time counter and combining	it with	some offset, regularly updated by
     tc_windup() on hardclock()	invocation.

     Different platforms provide different kinds of timer hardware.  The goal
     of	the time counters subsystem is to provide a unified way	to access that
     hardware.

     Each driver implementing time counters registers them with	the subsystem.
     It	is possible to see the list of present time counters, via the
     kern.timecounter sysctl(8)	variable:

     kern.timecounter.choice: TSC-low(-100) HPET(950) i8254(0) ACPI-fast(900) dummy(-1000000)
     kern.timecounter.tc.ACPI-fast.mask: 16777215
     kern.timecounter.tc.ACPI-fast.counter: 13467909
     kern.timecounter.tc.ACPI-fast.frequency: 3579545
     kern.timecounter.tc.ACPI-fast.quality: 900
     kern.timecounter.tc.i8254.mask: 65535
     kern.timecounter.tc.i8254.counter:	62692
     kern.timecounter.tc.i8254.frequency: 1193182
     kern.timecounter.tc.i8254.quality:	0
     kern.timecounter.tc.HPET.mask: 4294967295
     kern.timecounter.tc.HPET.counter: 3013495652
     kern.timecounter.tc.HPET.frequency: 14318180
     kern.timecounter.tc.HPET.quality: 950
     kern.timecounter.tc.TSC-low.mask: 4294967295
     kern.timecounter.tc.TSC-low.counter: 4067509463
     kern.timecounter.tc.TSC-low.frequency: 11458556
     kern.timecounter.tc.TSC-low.quality: -100

     The output	nodes are defined as follows:

     kern.timecounter.tc.X.mask	is a bitmask, defining valid counter bits,

     kern.timecounter.tc.X.counter is a	present	counter	value,

     kern.timecounter.tc.X.frequency is	a counter update frequency,

     kern.timecounter.tc.X.quality is an integral value, defining the quality
     of	this time counter compared to others.  A negative value	means this
     time counter is broken and	should not be used.

     The time management code of the kernel automatically switches to a
     higher-quality time counter when it registers, unless the
     kern.timecounter.hardware sysctl has been used to choose a	specific de-
     vice.

     There is no way to	unregister a time counter once it has registered with
     the kernel.  If a dynamically loaded module contains a time counter you
     will not be able to unload	that module, even if the time counter it con-
     tains is not the one currently in use.

SEE ALSO
     attimer(4), eventtimers(4), ffclock(4), hpet(4)

BSD				August 12, 2015				   BSD

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO

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